A treadmill is a great tool. If you have a dog-powered mill, it can be a good form of exercise. The motorized treadmills are good exercise for smaller dogs, and those with more modest exercise requirements.
A dog like mine with "big wheels" just can't get enough exercise on a motorized mill, unless you have the speed and incline turned up to where the dogs safety is at risk. At top speed, one misstep would put the dog through the wall. One benefit, however, is the focus a treadmill demands from the dog. If he gets distracted, he starts going backward and he self-corrects. My dog doesn't like going backward. The demand for the dog's focus seems to have a positive effect on his attitude, and seems to trim his sails as much or more than the physical workout. 30 minutes at an easy trot just won't scrub off a whole lot of energy.
If you have a large breed dog, you'll want a treadmill with a long bed. We got a "health club quality" treadmill with a big track bed. The quieter motors of the better units are an advantage, too.
As a side note: never pay retail for exercise equipment. Always buy second-hand. People buy these things, use them a few times, and then they become pieces of ugly furniture with clothes draped over them, and boxes piled on the track bed.